SACRAMENTO, Calif.  The U.S. Fish and Wildlife Service announced on Thursday that it reduced its protected habitat designation for the California red-legged frog to about 450,000 acres -- down 40 percent from its previous proposal and a number environmentalists decried as far too low.

Federal officials, however, also said an important component of the final designation was a rule that strongly encourages ranchers, who have populations of the threatened frog on their land, to make their operations more compatible with the animal's needs. The rule applies to than 1 million acres that may not be within the area designated as habitat that's critical for the species' survival.

Man-made ponds built so cattle can access drinking water, for example, can be used by the frogs as breeding ground, officials said.

The frog, the largest species native to the western United States, is believed to have inspired Mark Twain's tale, "The Celebrated Jumping Frog of Calaveras County."

The agency said it reduced the critical habitat to 450,288 acres in 20 counties -- from November's proposal of 737,912 acres in 23 counties -- because updated information about the frogs' habitat needs and better maps allowed it to better pinpoint which areas needed designation. It also cited an economic analysis showing that proposal would have cost $497 million in lost development opportunities over 20 years.

Environmental groups accused the agency of allowing developers and other special interests to slash the frogs' critical habitat from the 4.1 million acres initially proposed in 2001.

"It's taken 10 years and several court orders for the Fish & Wildlife Service to fulfill its legal obligation to designate critical habitat for the California red-legged frog, and still the agency didn't get it right," Mike Sherwood, an attorney with Earthjustice, said in a statement.